A perpendicular magnetic recording system is a system in which the easy axis of magnetization of a magnetic recording layer, that has hitherto been laid in the in-plane direction of a medium, is laid in the perpendicular direction of the medium. Whereby, the demagnetizing field in the vicinity of the magnetization transitional region constituting the boundary between recording bits is decreased and, therefore, the magnetostatic stability is increased and the resistance to thermal fluctuation is enhanced as the recording density increases. Accordingly, the perpendicular magnetic recording system is a suitable system for improving the areal recording density.
A perpendicular magnetic recording medium is composed of a non-magnetic substrate, and a backing layer (a soft magnetic layer), an under layer, an intermediate layer and a perpendicular magnetic recording layer that are sequentially laminated on the non-magnetic substrate. When the backing layer made of a soft magnetic material is provided between the non-magnetic substrate and the perpendicular magnetic recording layer, the perpendicular magnetic recording medium is capable of functioning as a so-called vertical two-layer medium and acquiring a high recording ability. In this case, the soft magnetic backing layer is fulfilling the role of refluxing the recording magnetic field from the magnetic head and is enabled to enhance the recording/reproducing efficiency.
The under layer is a governing element that determines the particle diameters and orientations of the intermediate layer and perpendicular magnetic recording layer formed thereon, and the selection of the material therefore proves to be very important in order to determine the recording/reproducing properties of the magnetic recording medium. Therefore, various materials used in the under layer have been proposed. For example, Ti alloys (see, for example, Patent Literature 1), NiFeCr alloys (see, for example, Patent Literature 2) and the like have been proposed. These materials can have an hcp structure, an fcc structure, an amorphous structure of Ti and the like.
The interposition of the under layer between the backing layer and the perpendicular magnetic layer causes the distance between the magnetic head and the surface of the backing layer to be increased by the thickness of the under layer. In this case, it becomes necessary to thicken the backing layer so as to perform sufficient writing. However, when the under layer is made of a material having a soft magnetic property, it becomes possible to control the role of the backing layer and the crystal orientation of the intermediate layer disposed thereon.
However, the construction of medium proposed heretofore is not sufficient to obtain a perpendicular magnetic recording medium having excellent recording/reproducing properties. Therefore, it has been desired to obtain a magnetic recording medium that solves this problem and allows manufacture at low cost.
Thus, the present applicant has proposed a perpendicular magnetic recording medium in which a soft magnetic film that constitutes a backing layer has an amorphous structure, and a NiW alloy is used in an under layer and a Ru alloy is used in an intermediate layer (see Patent Literature 3).